The concept of squelch is to automatically quiet or mute a radio receiver in response to a specified input signal characteristic and circuits for providing this operation are well known in the art. In high gain receivers the speaker noise produced by the absence of carrier on a given frequency can be very annoying to an operator. Normally such squelch circuits continuously monitor for a presence of an RF signal or carrier. When no signal is detected the squelch circuit is activated and the audio channel becomes inoperative.
One of the principal difficulties with such detection systems is that they require constant monitoring for the detection for the presence of a carrier and can cause a substantial power drain. When applied to a paging receiver system with a large number of pagers with a correspondingly high message traffic rate, the transmitter carrier signal may never be shut down because of the volume of traffic. Therefore the absence of a carrier would never be detected so that such squelch circuits could not operate correctly. A further difficulty is that the usual analog circuitry to achieve squelch requires some period of time to confirm detection and activation. Thus there frequently occurs a tailing off of the audio channel. The deactivation control word of the present invention operates to selectively squelch the paging receiver which has been selectively addressed. Thus, only the pager which has been correctly addressed and received the message undergoes deactivation.
The use of a deactivation control word for achieving this function with respect to a paging system has the distinct advantage that not only can the voice channel and thus the operator reception be clear of the annoying noise sound but it permits messages to be more tightly packed and thus increase the message throughput.
In the system of the preferred embodiment, the coding format to achieve paging is digital in nature and therefore the coding signal which is utilized to provide the deactivation control is itself a digital signal. Thus the squelch operation of the paging receiver has the same sensitivity as for normal the paging. The use of a digitally encoded word to achieve squelching in such a circuit provides the many advantages. The use of a positive deactivation control word at the conclusion of a variable length audio message provides utilization of the now wanted normal time out allocated for fixed length message systems and allows a more tightly packed message arrangement, thus increasing throughput for the entire system.